| Summary: | © 2017 by Taylor & Francis Group, LLC. Autism spectrum disorder (ASD) is a neurodevelopmental condition of multifactorial origin. Aside from clinically relevant ASD phenotypes qualifying for a categorical diagnosis, data increasingly support the notion of broader/extended phenotypes and traits of ASD that are continuously distributed in the general population, with variation in ASD traits determined by factors overlapping with the clinical phenotypes. Earlier twin studies confirmed repeatedly the genetic causes for ASD, with only modest environmental influences. Thus, ASD is considered one of the most heritable of psychiatric conditions with a heritability estimates between 80% and 90%. However, recent twin studies have suggested a stronger environmental influence on ASD etiologies. This chapter gives a short theoretical background on the basics of behavioral genetics and twin designs, summarizes the contribution of twin research to the knowledge of ASD etiologies, and discusses areas of interest for future twin research within the field of ASD.
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